In the arena of complex integrated circuits, there are sometimes portions of circuits that require voltage references for proper functioning. A voltage reference provides a precise output voltage, one that is much more accurate than can be produced by a voltage regulator. Its output voltage is compared to other voltages in a system and, usually, adjustments are made to those other voltages based on the reference difference. References are similar to regulators in how they function, but they are used much differently. While regulators are used to deliver power to a load, references are normally used with a small, stable load (if any) to preserve their precision. Only a few of the existing reference designs have the capability to deliver a load greater than a few milliamps while maintaining a precision output voltage. A reference is not used to supply power but to provide a system with an accurate analog voltage for comparison purposes. The band-gap reference circuit has long been used in integrated circuits for that purpose.
A band-gap reference takes advantage of the electro-chemical properties of a material. In a semiconductor, the amount of energy which allows the material to become conductive, i.e. move current in the presence of a voltage, is known as the band gap energy. The band gap energy is different for a variety of materials. However, silicon, the foundation material for a preponderance of integrated circuits, has a predictable band-gap energy that changes little with temperature over most of the temperature range of normal integrated circuit operations.
The band-gap reference is widely used in almost every application of IC technology. One common method of band-gap implementation is use of current generated by the delta Vbe of a pair of unijunction transistors which essentially function as diodes. The current then flows through a diode chain to achieve a constant reference band-gap voltage. A significant problem with such simple reference circuits is a high output impedance which can change the reference behavior if the band-gap reference circuit were connected to a high noise stage.
Some early band-gap reference circuits used conventional junction-isolated bipolar-IC technology to make relatively stable low-voltage references. This type of reference became popular as a stable voltage reference for low-voltage circuits, such as in 5-volt data acquisition systems where zener diodes were not suitable.
A common failing in band-gap reference circuits, as mentioned above, is a characteristically high impedance that results in a noisy circuit. Because the demands on a reference get ever tighter with higher precision circuits, a stable low-noise performance is crucial.
Another common failing of band-gap circuits is the requirement for a relatively high VCC, substantially higher than the reference voltage. Since a band-gap voltage is almost always very close to 1.2 volts, a minimum value for VCC is usually somewhere around 2 volts. Since modern digital ICs using 1 volt technology are becoming daily more common, the requirement for a higher VCC can be a design limitation.
What is needed, then, is a band-gap reference circuit that has an innate low impedance to allow for stable low-noise operation. A further need exists for a band-gap reference circuit that can produce a usable reference voltage while being powered by a low supply voltage.